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The pneumatocele’s likely cause, an MRI would show, was an osteoma, or benign bone tumor, that had formed in the man’s sinus and was eroding through the base of the skull, Brown said.

Its development from fibrous tissue is more likely to result in a fibroma; from fat tissue, a lipoma, or a myxoma; from cartilage or bone, a chondroma or osteoma.

Less frequently fibroma, osteoma, and parasitic, hæmorrhagic, and other cysts are met with.

Roentgenray examination may show the shadow of enchondromata or osteomata, and will also show variations in aeration should the tumor be in a bronchus.

Osteoma.—The true osteoma is composed of bony tissue, and originates from the skeleton.